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(Kena Betancur/AFP/Getty Images)

It looks like more customers have been steering clear of Chipotle, as health concerns linger in the wake of E. coli and norovirus incidences that have sickened patrons across the country.

In December, Chipotle's comparable sales plunged 30%, the Mexican restaurant chain said on Wednesday in a regulatory filing.

Chipotle now expects its fourth quarter comparable sales to decline 14.6%, worse than the 8% to 11% fall it had previously projected. This is a reflection of the fact that things have only been getting worse for Chipotle, which saw sales fall just 16% in November.

Chipotle also said on Wednesday it has been served with a federal grand jury subpoena in California, relating to a criminal investigation of an August norovirus incident at one of its restaurants in Simi Valley. The investigation is being carried out in conjunction with the U.S. Food and Drug Administration’s Office of Criminal Investigations.

Since October, Chipotle has been working to implement new food safety procedures, with co-CEO Steve Ells pledging to make the Mexican restaurant chain ”the safest place to eat.”

Doing so is expensive. The company said it expects to have incurred $14 million to $16 million in one-time expenses during the quarter, including the cost of lab testing food samples, hiring food safety experts and replacing food in certain restaurants.

Rising costs have pressured its bottom line. For the fourth quarter, Chipotle now anticipates earnings in the range of $1.70 to $1.90 per share. Previously it had forecast $2.45 to $2.85 per share, which even then was well below the $4.09 analysts had been anticipating.

Chipotle's stock has dropped nearly 40% since health concerns surfaced, and fell another 3% in pre-market trading on Wednesday to $436.09.